1. Field of the Invention
The present invention relates to a method and apparatus for improving configurations of serving grants for a user equipment in a wireless communications system, and more particularly to a method and apparatus for improving configurations of serving grants for a user equipment in a high speed uplink packet access (HSUPA) system with 16 quadrature amplitude modulation (16 QAM) or higher order modulation capability.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting third generation mobile telecommunication technology, the prior art provides High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), which are used to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
HSUPA increases upstream network performance, reduces transmission delay by rapid retransmission of erroneous data transmissions, and can adjust transmission rate based on channel quality. To realize this type of “power control,” HSUPA adopts technologies such as NodeB Scheduling, Hybrid Automatic Repeat Request (HARQ), Soft Handover, and Short Frame Transmission. Correspondingly, the 3rd Generation Partnership Project (3GPP) defines an Enhanced Dedicated Transport Channel (E-DCH) for controlling operations of HSUPA. E-DCH introduces new physical layer channels, such as E-HICH, E-RGCH, E-AGCH, E-DPCCH, and E-DPDCH, which are used for transmitting HARQ ACK/NACK, Uplink Scheduling Information, Control Plane information, and User Plane information. Detailed definitions of the above can be found in the Medium Access Control (MAC) protocol specification, “3GPP TS 25.321 V7.3.0,” and are not given here.
Through Node B Scheduling technology, a base station (Node B) is allowed to adjust transmission power, or Serving Grant (SG), of mobiles or user equipments (UEs) within its transmission range (cell), so as to control uplink transmission rate of the UEs. Referring to the aforementioned MAC protocol specification, the network can provide Relative Grant (RG) messages and Absolute Grant (AG) messages to a UE through an E-DCH Relative Grant Channel (E-RGCH) and an E-DCH Absolute Grant Channel (E-AGCH) respectively, so as to adjust the transmission grant of the UE.
The RG messages adjust the SG of the UE according to a state variable reference_ETPR (which is the previous SG value). Detailed description of the variable reference_ETPR can be found in the aforementioned MAC protocol specification, and is not given here. The RG messages can be divided into Serving Relative Grant (SRG) messages and Non-Serving Relative Grant (NSRG) messages. An SRG message is transmitted from a serving cell of a serving Radio Link Set (RLS), and allows a Node B scheduler to incrementally adjust the SG of a UE under its control through “UP”, “DOWN” and “HOLD” commands. In addition, only one SRG command may be received at any one time. An NSRG message is transmitted from a non-serving Radio Link (RL), and allows neighboring Node B's to adjust the transmission rate of UEs that are not under their control in order to avoid overload situations. By definition, there could be multiple non-serving relative grant commands received by the MAC at any time. NSRG commands include “DOWN” and “HOLD.”
Furthermore, a scheduling grant table (SG-table) is configured for a UE by upper layers to perform a serving grant update procedure when a RG message is received. The serving grant update procedure converts a previous SG value stored in the state variable reference_ETPR to a scheduled grant index in the SG-table, so as to set an SG value that the UE is allowed to use in the next transmission through commands carried by the RG message. According to the aforementioned MAC protocol specification, when the serving grant update procedure is performed, the UE shall determine a scheduled grant value larger than or equal to the previous SG value, i.e. the state variable reference_ETPR, in the SG-table and also determine a corresponding scheduled grant index as a basis to adjust the SG value of next transmission.
In comparison, the AG messages are used to adjust the transmission grant of the UE directly, and can be classified into two types: Primary and Secondary. A Primary AG message provides an uplink resource grant for a specified UE served by a cell. A Secondary AG message provides an uplink resource grant for a group of UEs served by the cell, so as to reduce signaling overhead. Similarly, an absolute grant value mapping table is configured in a UE by upper layers as well, which is utilized for mapping and setting the SG value of the next transmission according to an absolute grant index carried by a received AG message.
Please note that, the 3rd Generation Partnership Project (3GPP) newly introduces an SG-table and an absolute grant value mapping table in the aforementioned MAC protocol specification to support a high speed uplink packet access (HSUPA) system with 16 quadrature amplitude modulation (16 QAM) capability, in which the range of SG values are enlarged for significantly enhancing data transmission rate.
However, since a maximum absolute grant value in the new absolute grant value mapping table is higher than a maximum scheduled grant value in the new SG-table, some unexpected errors may occur when the UE receives a RG message and performs the serving grant update procedure. For example, when the UE receives an AG message with the maximum absolute grant value in some transmit time interval (TTI), the UE can then set the SG value of the next transmission to be the maximum absolute grant value (which is (377/15)2×4 in this case) and stores the used SG value (for example, the maximum absolute grant value) into the state variable reference_ETPR. After that, when a RG message (generally indicating “DOWN”) is received, the UE shall perform the serving grant update procedure to correspondingly adjust the SG value of the next transmission. However, since the previous SG value stored in the variable reference_ETPR exceeds the maximum value of the new SG-table (which is (376/15)2×4 in this case), the UE cannot determine a scheduled grant value larger than or equal to the previous SG value and a corresponding scheduled grant index from the SG-table, so that the SG value of the next transmission cannot be set correctly, resulting in software errors.